1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine and the like, and more particularly, it relates to a multi-color image forming apparatus capable of forming a multi-color image
2. Related Background Art
Recently, a multi-color image obtained by composing different color visualized images or different informations on a sheet has been proposed, and, to this end, multi-color image forming apparatuses having a plurality of developing devices have been put on the market.
Among these multi-color image forming apparatuses, there has widely been proposed a multi-color image forming apparatus in which multi-color development is effected by using two or more developing devices during one revolution of an image bearing member and the developed different color images are correctively transferred onto a transfer material (for example, sheet). For example, U.S. Pat. Nos. 4,572,651 and 4,416,533 disclose a technique in which development is effected by two developing devices while keeping electric fields thereof constant by using DC bias. However, these patents mainly discuss a method for forming a latent image and do not teach problems caused in the development
On the other hand, U.S. Pat. No. 4,349,268 and the Japanese Patent Application Laid-Open No. 56-144452 (1981) disclose a technique in which a first color visualized image (toner image) is prevented from being distorted by sliding contact with second color developer by using a non-contact developing method for applying AC developing bias to the second color developer, and the Japanese Patent Application Laid-Open No. 56-12650 (1981) discloses a technique in which a first color visualized image is prevented from being distorted by sliding contact with second color developer by using a non-contact developing method for applying DC developing bias to the second color developer. Incidentally, the above-mentioned Japanese Patent Application Laid-Open No. 56-144452 does not describe potential of the first color toner image.
In this way, conventionally, in the multi-color image formation, although it is well-known to effect next color development without distorting the previously formed toner image, in order to achieve the same purpose, it is also known to increase potential level of a latent image for a toner image to be firstly developed. For example, U.S. Pat. No. 4,460,961 discloses a technique in which, after a first color toner image is formed, by charging an entire surface of an image bearing member with polarity same as that of the toner to bring potential of a latent image for the first color toner image (surface potential of a portion of the image bearing member on which the first color toner image is formed) to be substantially equal to potential of a non-developed portion (non-image portion), second color development can be effected without distorting the first color toner image. This technique is called as "negative-negative re-charging system" in a one-pass multi-color image forming apparatus and has recently been investigated widely.
However, the above-mentioned conventional negative-negative re-charging system arises the following problems.
After the first color toner image is formed, when the image bearing member is charged with the same polarity as that of the toner by means of a second color charger (re-charger), the first color toner on the image bearing member is scattered toward and adhered to a shield and a grip portion of the re-charger, thereby smudging the latter. The toner contamination on the shield and the grip portion is gradually accumulated as the copying operation is repeated, with the result that a corona wire of the re-charger is contaminated by toner, thereby causing uneven charging of the image bearing member due to poor re-charging of the re-charger.
Consequently, the potential of the latent image form the first color toner image does not become substantially the same as the potential of the non-image portion, with the result that the first color toner image is distorted during the second color development or the toner is adhered to other portions rather than a second color latent image. Further, since the first color developer enters into the second color and other developing devices, the first color developer is gradually used in the second color and other developing devices during the image formation, thereby making an image obscure.